The present invention relates to a method and to an apparatus for calibrating tubular articles consisting of a pulverulent metallic material which has undergone isostatic compression and a subsequent sintering operation. It is to be noted that while the invention is considered to be of special value in connection with the calibration of linings for the cylinders of internal combustion engines, its general field of use is much broader and covers all tubular articles manufactured as stated above.
In the Prior Art a tubular article of the type above specified, including cylinder linings, are calibrated by placing the lining around a cylindrical mandrel. For obvious reasons it is then necessary that the internal diameter of the lining exceeds the diameter of the mandrel by an amount sufficient to permit convenient mounting of the lining thereon. The calibration process involves subjecting the lining to radially inwardly acting forces striving at bringing the internal surface of the lining into intimate and accurate contact with the cylindrical surface of the mandrel. As is understood, this means that the internal diameter of the lining must be reduced by an amount corresponding to the initial clearance between the mandrel and the lining. This forced deformation of the lining generates inside its material reactional forces in the form of tensions tending to counteract the deformation. The tangible result of the presence of such internal stresses in the material may occur in two different forms which may be present separately or in combination.
A cylinder lining not yet calibrated generally exhibits significant irregularities in terms of wall thickness, diameter and circular form. If the inner diameter substantially exceeds the diameter of the mandrel, the lining cannot be deformed into intimate contact with the mandrel all around its circumference. Instead, within a greater or smaller sector the lining will bulge out from the mandrel or, differently put, a "fold" is formed. If, in addition thereto, one or both of the other two just-mentioned irregularities is present, then this undesired deformation is accentuated. Should the wall thickness of the lining be small at one place in the circumferential direction, this means an indication for the formation of a fold in that area. Correspondingly, if the lining blank when introduced into the calibration press deviates from a true circular form, this deviation will be amplified when the clearance between the mandrel and the lining is too great as just mentioned. It should also be observed that the lining blank may also have such variations in the direction of its longitudinal axis which may provoke local fold formations at different levels or, alternatively, that such folds become differently pronounced at different levels.
It has been found that even in such cases when the irregularities of the lining blank are not sufficiently great to cause the formation of folds when the lining is still surrounding the mandrel, they may be great enough to retain inside the material stresses which show up at a later stage, when the lining has been unloaded of the compression forces acting inside the press and has been removed from the mandrel. In either of the cases above discussed it will apparently be necessary to subject the calibrated lining also to a machining operation in order to ensure that its final dimensions will fall within the tolerance limits.
It is a primary object of the present invention to provide a method and an apparatus for calibrating tubular articles, especially linings for the cylinders of internal combustion engines, in such a way that the risk of a creation or continued presence of irregularities of the type here discussed is greatly reduced. For the sake of completeness and to avoid any misunderstanding it should be added that the term "fold" as used herein is not meant to be construed literally. In the context of the present invention the term just specifies that, within a certain portion of the circumference of the lining, there is a lack of complete contact between the surfaces of the lining and the mandrel.